1. Field of the Invention
This invention relates to webs or backings which are reinforced with organic synthetic thermoplastic yarns, are ultimately utilized in products requiring a combination of good elongation resistance with sufficient flexibility to accommodate to repeated curving and flexing, and are exposed to heat either in ultimate use or during manufacture of the product in which they are ultimately used. Examples of such products include conveyor and power transmission belts, hoses, skirts for air supported vehicles such as marine hovercraft, shells for air supported structures, and backings for coated abrasives to be used in belt form. This invention is particularly related to the latter group of products. The synthetic yarns may be present in the webs as individual yarns or in the form of fabrics. The invention is more particularly related to coated abrasives reinforced in one of the manners described in U.S. application Ser. No. 06/420,466 by Yarbrough, filed Sept. 20, 1982, the specification of which is incorporated herein by reference.
2. Description of the Prior Art
Excessive stretch or elongation of backings which depend on synthetic thermoplastic textile yarns for their primary strength members has been a serious concern ever since these synthetic yarns began to be used to replace cotton as reinforcing yarns. Various expedients, primarily heat-stretching, have been used to ameliorate the problem and are well known in the various arts in which the problem has been encountered. With respect to coated abrasives, some of the first practical means of reducing stretch were described in U.S. Pat. Nos. 4,035,961 and 4,140,829 to Pemrick and Cocanour. While the methods described in these patents were substantial improvements over the prior art at the time and provided products which are satisfactory for many applications of coated abrasives, as evidenced by continuing commercial sales, there are some applications for which the stretch resistance achieved by these methods is not fully satisfactory.
A more recent development, which is especially useful in combination with the methods of Pemrick and Cocanour and for backings in which continuous filament synthetic yarns provide the primary elongation resistance, is described in U.S application Ser. No. 06/334, 710 by Ibrahim, now U.S. Pat. No. 4,396,657 and is incorporated herein by reference. This Ibrahim method also advanced the prior art but has proved not to be entirely satisfactory in all cases. For applications requiring truly extraordinary stretch resistance, especially when substantial heat is generated, still further improvement is indicated.
Most workers in the prior art have relied on measurements at room temperature to characterize the adequacy of elongation resistance, and the most common measurement has been a conventional single tensile test per web, backing, or product sample on a laboratory tensile and elongation test machine. In part at least, this method of testing has reflected a strong tendency in the prior art to regard conventional heat setting and/or stretching as "permanently" stabilizing yarns against shrinkage at any temperature significantly lower than that of prior heat setting and/or stretching.